ShardId BalancerPolicy::_getMostOverloadedShard(const ShardStatisticsVector& shardStats,
const DistributionStatus& distribution,
const string& chunkTag,
const set<ShardId>& excludedShards) {
ShardId worst;
unsigned maxChunks = 0;
for (const auto& stat : shardStats) {
if (excludedShards.count(stat.shardId))
continue;
const unsigned shardChunkCount =
distribution.numberOfChunksInShardWithTag(stat.shardId, chunkTag);
if (shardChunkCount <= maxChunks)
continue;
worst = stat.shardId;
maxChunks = shardChunkCount;
}
return worst;
}
bool BalancerPolicy::_singleZoneBalance(const ShardStatisticsVector& shardStats,
const DistributionStatus& distribution,
const string& tag,
size_t imbalanceThreshold,
vector<MigrateInfo>* migrations,
set<ShardId>* usedShards) {
const ShardId from = _getMostOverloadedShard(shardStats, distribution, tag, *usedShards);
if (!from.isValid())
return false;
const size_t max = distribution.numberOfChunksInShardWithTag(from, tag);
if (max == 0)
return false;
const ShardId to = _getLeastLoadedReceiverShard(shardStats, distribution, tag, *usedShards);
if (!to.isValid()) {
if (migrations->empty()) {
log() << "No available shards to take chunks for tag [" << tag << "]";
}
return false;
}
const size_t min = distribution.numberOfChunksInShardWithTag(to, tag);
if (min >= max)
return false;
const size_t totalNumberOfChunksWithTag =
(tag.empty() ? distribution.totalChunks() : distribution.totalChunksWithTag(tag));
size_t totalNumberOfShardsWithTag = 0;
for (const auto& stat : shardStats) {
if (tag.empty() || stat.shardTags.count(tag)) {
totalNumberOfShardsWithTag++;
}
}
// totalNumberOfShardsWithTag cannot be zero if the to shard is valid
invariant(totalNumberOfShardsWithTag);
invariant(totalNumberOfChunksWithTag >= max);
// The ideal should be at least one per shard
const size_t idealNumberOfChunksPerShardWithTag =
(totalNumberOfChunksWithTag < totalNumberOfShardsWithTag)
? 1
: (totalNumberOfChunksWithTag / totalNumberOfShardsWithTag);
const size_t imbalance = max - idealNumberOfChunksPerShardWithTag;
LOG(1) << "collection : " << distribution.nss().ns();
LOG(1) << "zone : " << tag;
LOG(1) << "donor : " << from << " chunks on " << max;
LOG(1) << "receiver : " << to << " chunks on " << min;
LOG(1) << "ideal : " << idealNumberOfChunksPerShardWithTag;
LOG(1) << "threshold : " << imbalanceThreshold;
// Check whether it is necessary to balance within this zone
if (imbalance < imbalanceThreshold)
return false;
const vector<ChunkType>& chunks = distribution.getChunks(from);
unsigned numJumboChunks = 0;
for (const auto& chunk : chunks) {
if (distribution.getTagForChunk(chunk) != tag)
continue;
if (chunk.getJumbo()) {
numJumboChunks++;
continue;
}
migrations->emplace_back(distribution.nss().ns(), to, chunk);
invariant(usedShards->insert(chunk.getShard()).second);
invariant(usedShards->insert(to).second);
return true;
}
if (numJumboChunks) {
warning() << "Shard: " << from << ", collection: " << distribution.nss().ns()
<< " has only jumbo chunks for zone \'" << tag
<< "\' and cannot be balanced. Jumbo chunks count: " << numJumboChunks;
}
return false;
}
